Using the leg file downloaded:
The file is a 3ds file and so must be imported rather than opened. once imported, move it to ground level and create a ground plane.
2. Go into the left viewport and start inserting dummys. (We use dummys rather than the objects themselves so we dont have to worry about object pivot points, and it makes it easier to import into other software.) Make a dummy at the ankle, knee and hip of the legs. now move them into position over one of the legs and copy them to the other. Draw another dummy at the centre of the pelvis- this will be our control dummy.
3. Attach the feet to the ankle dummys, then for safety, freeze the feet. link the calves to the knee dummys, and freeze the calves. Finally link the thighs to the hip dummys, and the pelvis to the control dummy. Always link TO THE DUMMY, to make the dummy the parent.
4. On the display tab hide the geometry of the file, so that only the dummys remain. Now link the dummys together- ankle to knee to hip to pelvis. Turn geometry back on to see the model again.
5. Select one of your hip dummys, then choose Animation>IK Solvers, HI Solver. There will be a dotted line from the dummy to your cursor. Now click the ankle dummy. A white triangle will appear between hip, knee and ankle, and a small blue cross will appear in the middle of the ankle dummy. This is the IK Solver itself. from now on, do not manipulate the model or the dummys, only animate the model using this solver. Do this for the other leg.
6. We notice now that if we move the hips down, the ankles stay in place but the tips of the feet go through the floor. In top viewport, draw a rectangle under each foot (Not a plane- a spline rectangle). Click on your ankle dummys and choose Animation>constraint> Orientation constraint. The dotted line will attach to your cursor- choose the rectangle under the foot to constrain the orientation of the ankle to that of the rectangle. You may need to click ‘keep initial offset’ in the modifier panel. Do this for the other leg.
7. Move the pivot point of the rectangles and align them to the pivot point of the ankle dummies. Now, select the IK solvers (the little blue crosses) in the ankles, and choose Animation> Constraints> Position constraints, and click the dotted line onto the rectangle. Do this for the other leg. Now freeze all your dummys. The model can now be controlleed by the rectangles at the feet, and the pelvis dummy.